The invention relates to a method, equipment and a recording device for suppressing pulsed interference in analogue audio and/or video signals, particularly for suppressing clicks during playback of old sound carriers.
1. Prior Art
Pulsed interference (called clicks below) can occur in voice and music signals originating from a wide variety of sources. These can be old sound carriers, such as old shellacs (gramophone records), LPs, audio tapes, audio or video cassettes, as well as film sound or multipath transmission paths. These offending clicks are usually removed in two steps: detection of the click and interpolation, i.e. filling in the click. A. known method of detection is the use of a permanently set threshold value above the peak value of the audio signal. One limitation of this method is that many clicks do not have a larger amplitude than the peak signal and therefore remain undetected. In the past, the clicks have been filled in by carrying out linear or else higher-order interpolation based on the sample values before and after the gap caused by the click. In this case, the signal is not reproduced in the gap. Quality is improved by using so-called random interpolation, in which a model of the audio signal is used.
Such a method is described in GB 2 217 902. Here, a mathematical model is initially calculated from the undistorted data before and after the click. This autoregressive model is then used to fill in the click. Specifically, this method takes the undistorted data before the click and uses the model to determine prediction values at the point of the click and after the click. The values at the point of the click are then changed in such a way that the error for the data after the click becomes minimal. A clear disadvantage of this method is the high complexity arising from the fact that the order of the model must correspond to about twice the number of sample values to be filled in. A further disadvantage is that the method does not work correctly beyond a certain click density, because there are then no longer sufficient undistorted data available for calculating the mathematical model. Furthermore, the method produces a noticeable delay which can be a disadvantage in certain cases.
2. Invention
The invention is based on the object of providing a method for suppressing pulsed interference in audio or video signals. This object is achieved by the method specified in claim 3.
The invention is based on the further object of providing equipment for suppressing pulsed interference in audio or video signals. This object is achieved by the equipment specified in claim 7.
Finally, the invention is based on the further object of providing a recording device for suppressing pulsed interference in audio or video signals. This object is achieved by the recording device specified in claim 1.
In principle, the essence of the method according to the invention for suppressing pulsed interference in analogue audio and/or video signals, sample values being formed from the analogue audio and/or video signals and being used as an input signal, and undistorted sample values before or after the interference being used to suppress the interference, is that
adaptive filtering is carried out and is used both for detecting and for interpolating the pulsed interference, in which case
the adaptive filtering of the input signal is used to determine an error signal,
a sample value in the input signal is designated as distorted if the absolute instantaneous value of the error signal exceeds a threshold value,
the output signal of the adaptive filtering is used as a substitute value for the sample value designated as distorted,
the sample value designated as distorted is also replaced in the input values of the adaptive filtering.
A significant difference from the method described in GB 2 217 902 is that a mathematical model (AR, autoregressive) is explicitly calculated in the latter case, based on the undistorted data (sample values) before the click. In contrast, the method according to the invention makes it possible to dispense with the complex calculation of a mathematical model. The use of an adaptive filter involves no direct mathematical signal modelling. Hence, the method according to the invention is remarkably less complex and thus permits real-time declicking. Furthermore, it can also be used with a high click density and can therefore be used as a decrackler in order to suppress crackling, i.e. high-density pulsed interference. This produces superb results in comparison to other methods, as audio testing has also shown.
In the method according to the invention, it is advantageous that no change is made in the signal path if the absolute sample value in the error signal is below the respective threshold value.
It is particular advantageous if the adaptation of the adaptive filtering is carried out only at the discrete instants. at which the sample value in the input signal is designated as undistorted.
In addition, the adaptation is preferably suspended if the sample value in the input signal has been designated as distorted.
It is likewise advantageous to continue the measurement of the power of the error signal and/or of the input signal only for sample values which have been recognized as undistorted.
Finally, it is particularly advantageous to match the threshold value dynamically to the level characteristic.
In principle, the essence of the equipment according to the invention for suppressing pulsed interference in analogue audio and/or video signals, to which equipment sample values formed from the analogue audio or video signals are fed as the input signal, undistorted sample values before or after the interference being used to be able to output an output signal with suppressed interference, is
an adaptive filter for detecting and interpolating the pulsed interference, having the sample values of the input signal fed to it and being used to determine an estimated value for a future sample value and, from this, an error signal,
a unit for determining the instantaneous value of the error signal,
a changeover switch which switches between the estimated value and a sample value designated as distorted.
It is advantageous in this case that the sample value designated as distorted is also replaced in the input vector of the adaptive filter.
In principle, the essence of the recording device according to the invention for suppressing pulsed interference in analogue audio and/or video signals and digitally recording these audio and/or video signals, is
an analogue/digital converter for forming sample values from the analogue audio and/or video signals,
declicking equipment according to claim 7, to which equipment the sample values for suppressing pulsed interference are fed,
recording equipment for recording on a digital data carrier, to which equipment the output signal of the declicking equipment is fed directly or after further processing.
Advantageously, the recording device additionally has a playback device for analogue data carriers, the analogue audio and/or video signals recorded on these analogue data carriers being fed to the analogue/digital converter during playback of the analogue data carriers.